The present invention relates to a process for the production of sized and/or wet-strength papers, paperboards and cardboards by means of aqueous radiation-curable dispersions containing at least one polymer, characterized in that cationic groups are present, the papers, paperboards and cardboards obtainable therewith, and compositions comprising suspended wood pulp, chemical pulp and/or cellulose, and aqueous radiation-curable polymer dispersions containing cationic groups.
The terms “paper, paperboard and cardboard” used herein include sheet-like pulps and formed products which are produced from fibrous cellulose materials which are derived both from natural and from synthetic sources. Sheet-like pulps and formed products which are produced from combinations of cellulosic and non-cellulosic materials which originate from synthetic substances, such as e.g. polyamide, polyester or polyacrylic resin fibres, and from mineral fibres, such as asbestos or glass, are also included.
In the processes for the production of paper, paperboard and cardboard, resin sizes, alkyl ketene dimers (AKD) or alkylated succinic anhydride (ASA) are employed as standard as pulp sizing agents (an overview is to be found inter alia in A. Pingel Keuth, Chem. Unserer Zeit, 2005, 39, p. 402-409; J. Blechschmidt, Taschenbuch der Papiertechnik, Carl Hanser Verlag, Munich, 2010, p. 228 et seq. and p. 300; R. Schumacher, Stand und Perspektiven des Einsatzes von Leimungsmitteln in der Papier-, Karton-und Papierindustrie, in: Leimen, Füllen und Färben von Papier und Karton, Papiertechnische Akademie, 1999, editors: H. G. Völkel, F. Bräuning).
Resin sizes are based on modified tree resins in combination with aluminium salts, which are suspended with the stock shortly before the headbox. Disadvantages of the use of resin sizes is the difficulty of controlling the sizing, since this takes place in an optimum manner only at a pH of 4.7, and the associated high paper waste, the low flexibility with respect to further paper additives and the low stability of the papers because of the sizing at an acid pH.
Sizing is predominantly carried out with alkyl ketene dimers (AKD) or alkylated succinic anhydride (ASA). AKD and ASA are hydrophobic chemicals which are converted into an aqueous dispersion with the aid of protective colloids, such as e.g. cationic starch or polyvinylamine (see also DE-A 1 19710616). Sizing via AKD and ASA is carried out at a neutral pH, which must be controlled exactly, in order to achieve optimum sizing (US-A 2006/0231223 [0026]). A disadvantage of the use of AKD and ASA in the production of paper, paperboard and cardboard is the limited storage stability of the AKD and ASA dispersions, since ASA and AKD are reactive substances, AKD in particular being readily hydrolysed (AKD dispersions are storage-stable for approx. 30 days under controlled conditions). Furthermore, the dispersions are highly viscous and the solids content is only 20 wt. %, so that the logistics and use are involved and cost-intensive.
Wet strength agents used nowadays in papermaking are based essentially on melamine resin or polyamidoamine-epichlorohydrin resin (PAAE resin). Both wet strength agents have the disadvantage that if paper waste is obtained, dry paper waste can be only poorly beaten again, i.e. returned to the headbox after comminution and re-suspension. Relatively large amounts of waste are accordingly easily obtained in the production of wet-strength paper.
In DE-A14436058, polyether-hydrophilized polyisocyanates are employed as wet strength agents in paper production. Such systems have a pot life and are processable only for minutes to hours. The reason for this is the reaction of the isocyanate with water, followed by degradation to the amine, which then adds on to still free isocyanate. The build up in molecular weight leads after a short time, the so-called “pot life”, to a very high, unacceptable viscosity.
U.S. Pat. No. 3,971,764 A1, WO-A197/45395 and EP-A10165150 describe cationically hydrophilized polyurethane dispersions as pulp sizing agents in papermaking. Sizing of the paper is effected during drying of the paper and is not uncoupled in time from the drying. Beating of paper waste sized in this way is difficult. At any rate, further pulp sizing agent must be added to the beaten paper waste, which can lead to further process problems since metering of the renewed addition of pulp sizing agent is difficult. Interfering substances are formed, which in turn necessitate the addition of further chemicals. Sized paper which has already dried can no longer be beaten again and must be disposed of.
Radiation-curable aqueous polyurethane (meth)acrylate dispersions are known as binders for radiation-curable lacquers. The applications EP-A1753531, EP-A2/A3 1106633 and EP-A 1 1958974 are representative of these. These describe nonionically, anionically or cationically hydrophilized polyurethane (meth)acrylates which are employed as aqueous dispersions in particular for wood lacquers. The coating/lacquering of paper, cardboard or paperboard is also described inter alia. The use of these dispersions for the production of sized and/or wet-strength paper, cardboard or paperboard is not disclosed. A lacquer is not to be equated with a pulp sizing agent and/or wet strength agent which is employed in paper production, since a lacquer is a coating composition which is applied thinly to objects and is built up to a closed, solid film by chemical and/or physical processes. It has a protective, decorative or functional aim. A pulp sizing agent and/or wet strength agent for production of sized and/or wet-strength papers, paperboards or cardboards, on the other hand, is mixed with the chemical pulp and/or wood pulp, is within the paper after the papermaking, accordingly does not form a closed film, possibly reacts with the cellulose fibre or is deposited on it in places, and has the function of hydrophobizing the cellulose fibres and imparting dimensional stability to the paper, the paperboard or the cardboard in the wet state.
Conventional processes known from the prior art using the known pulp sizing agents and/or wet strength agents have disadvantages in the process of paper production: precise control of the reaction parameters (e.g. pH), lack of storage stability of the dispersions employed and lack of re-beatability of the paper waste obtained, i.e. being returnable to the headbox again after comminution and suspension.
The object was to provide a novel improved process for the production of sized and/or wet-strength paper, paperboard and cardboard which overcomes the disadvantages mentioned. Furthermore, the dispersions employed as pulp sizing agents and/or wet strength agents in the process according to the invention should have good retention properties, i.e. should be absorbed efficiently on to the cellulose fibres, and should be usable in various formulations and under variable conditions (e.g. temperature, concentration). In particular, re-beatability of the paper waste obtained should be achieved, i.e. the paper waste should be returnable to the headbox again after comminution and suspension. Furthermore, the dispersions employed in the process according to the invention should be of low viscosity and have a higher solids content than the AKD or ASA dispersions conventional hitherto.
It has been found, surprisingly, that in the process according to the invention an aqueous radiation-curable dispersion containing at least one polymer, characterized in that the polymer contains cationic groups, is outstandingly suitable for the production of sized and/or wet-strength paper, paperboard and cardboard and for hydrophobizing cellulose fibres. In the process according to the invention, the sizing or hydrophobizing action is achieved only after the radiation curing of the already dried paper. This has the advantage that paper, paperboard and cardboard which has already been dried but not yet subjected to radiation-curing can be beaten again, i.e. the paper waste does not have to be disposed of but can easily be fed back to the papermaking process. As a result, the process according to the invention is more flexible than the processes known hitherto.
It was known hitherto only that closed films of aqueous radiation-curable binders can be cured by means of radiation. It would therefore have been expected that a high absorption of the high-energy radiation by the chemical pulp or wood pulp would take place, so that no sizing by irradiation takes place in the process according to the invention. It has been found, surprisingly, that a sizing and/or wet strengthening of the paper, the paperboard and the cardboard is achieved by irradiation.